This invention relates to working substances used in laser isotope separation of silicon and methods of laser isotope separation of silicon utilizing the same working substances.
Optical absorption in the infrared region from 10.sup.2 to 10.sup.3 cm.sup.-1 is due to change in molecule vibration energy. In the absorption, isotope effects are sometimes remarkably large. When molecules including a particular isotope are irradiated with light having a wavenumber near that of a large absorption band of the molecule, the molecules are selectivly excited. As a result, it becomes possible to induce a chemical reaction on the molecules and the particular isotopes are separated from other isotopes. However, normal molecules do not cause the chemical reaction by absorbing a single photon having a wavenumber within the above wavenumber region because energy of only the single photon is not enough to cause the chemical reaction. On the other hand, when the molecules are irradiated with strong infrared laser rays the molecules absorb up to dozens of photons and cause decomposition. This decomposition is called as infrared multiple-photon decomposition.
Natural silicon consists of isotopes of mass numbers 28, 29 and 30 in the abundance ratio of [.sup.28 Si]: [.sup.29 Si]:[.sup.30 Si]=92.23:4.67:3.10. The isotope separation of silicon by means of infrared multiple-photon decomposition has been scarecely investigated. Only experiment concerning enrichment of .sup.29 SiF.sub.4 and .sup.30 SiF.sub.4 in which SiF.sub.4 was used as the working substance along with a carbon dioxide laser has been reported (J. L. Lyman and S. D. Rockwood; J. Appl. Phys., Vol. 47, No. 2, P. 595-601, (1976)).
However, the selectivity obtained by this experiment was very low. That is, concentrations of .sup.29 Si and .sup.30 Si were increased by only about 5%. Accordingly it is difficult to consider that the experiment can be applied to practical use. Demand for the silicon isotopes is increasing in the fields of medicine and agricultural chemicals and development of materials for electronic devices, so a method for high yield isotope separation of silicon is desired.
It is therefore an object of the present invention to provide working substances for use in the laser isotope separation of silicon and a method of laser isotope separation of silicon utilizing the same working substances which separates silicon isotopes in high yield.